By Alister Doyle
OSLO (Reuters) - Sea ice around Antarctica shrank in a warm period more than 100,000 years ago, an indication that man-made climate change could also trigger an abrupt retreat, a scientific report said Tuesday.
A thaw would reverse a paradoxical expansion of the extent of floating ice on the ocean around the frozen continent in recent decades, apparently defying the trend of global warming blamed on greenhouse gases.
Winter sea ice around Antarctica shrank 65 percent in a natural warm period between Ice Ages about 128,000 years ago, when temperatures were slightly warmer than now, according to the report in the journal Nature Communications.
The conclusions were based on ancient ice cores drilled from deep in the Antarctic ice sheet. The chemistry of snow indicates how far it blew from the ocean before it landed and got compressed into ice.
The ancient shrinking of the ice helps underpin forecasts by a panel of U.N. climate experts that global warming will mean a 58 percent retreat in Antarctica's sea ice by around the year 2200, the British scientific team that wrote the report concluded.
"A major reduction at 128,000 years ago could indicate a tipping point in the sea ice system," they wrote.
Lead author Max Holloway of the British Antarctic Survey (BAS) told Reuters that the ancient shrinking of sea ice may have preceded a collapse of an ice sheet in West Antarctica that spilled into the sea and pushed up sea levels.
"With a major reduction in sea ice, you'd expect the ice sheets to be more sensitive," he said. Separately, in 2014 another study suggested that some glaciers in West Antarctica have already begun an irreversible thaw.
Sceptics who reject mainstream scientific findings about global warming often point to the expansion of sea ice around Antarctica as evidence. Sea ice in the Arctic at the other end of the planet has been shrinking in recent decades.
Scientists say the contradictory trends may partly be because Antarctica is a continent surrounded by an open ocean, while the Arctic is an ocean surrounded by land - mainly Russia, Canada, Greenland and Alaska.
"By uncovering, for the first time, a huge retreat around Antarctica, we have established that sea ice in the Southern Hemisphere is also susceptible to major climate changes," co-author Louise Sime of BAS said in a statement.
(Editing by Jeremy Gaunt)